Textural, Mineralogical and Microfacies Characteristics of the Lower Paleogene Succession at the Nile Valley and Kharga Oasis Regions, Central Egypt
Total Page：16
File Type：pdf, Size：1020Kb
Journal of Geosciences, Osaka City University Vol. 47, Art. 4, p. 39-53, March, 2004 Textural, Mineralogical and Microfacies Characteristics of the Lower Paleogene Succession at the Nile Valley and Kharga Oasis Regions, Central Egypt 1 Ibrahim Mohamed GHANDOUR1,2, Abd EI-Monem T. ABD EL-HAMEED , 2 Mahmoud FARIS 1, Akmal MARzoUK I and Wataru MAEJIMA I Department of Geology, Faculty of Science, Tanta University, 31527, Tanta, Egypt 2Department of Geosciences, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan Abstract Four lithostratigraphic successions covering the Early Paleogene time have been measured and sam pled at the Nile Valley and Kharga Oasis areas in central Egypt. They include the Early-Late Paleocene upper Dakhla Shale and the Tarawan Chalk, and the Late Paleocene-Early Eocene Esna Shale. There is no marked variation between the grain size distribution of the Dakhla and Esna shales. They are generally composed of medium and coarse silts with small amounts of sand and clay. XRD analysis revealed the dominance of smectite with relatively low contents of kaolinite and illite and sporadic occur rence of palygorskite. Smectite was formed under warm semi - arid climatic conditions. Kaolinite, on the other hand, was formed under tropical to subtropical humid conditions. The relative proportions of smectite to kaolinite are controlled by the hydraulic sorting which concentrates kaolinite in the nearshore and shallow areas and drifts the smectite to deeper settings. The non-clay minerals are dominant by calcite with lower contents of quartz and local occurrences of feldspars. The high calcite content is consistent with the abundant calcareous fossils in the studied sediments. Dakhla Shale contains high carbonate contents and this is attributed to the high carbonate productivity and reduction of the detri tus influx during a period of sea level highstand. On the other hand, low carbonate contents of the Esna Shale was attributed to the dilution by high terrigenous influx during sea level lowstand. Petrographic examination of the Tarawan Chalk sediments and the interbedded limestones within the Dakhla Shale has led to the identification of four carbonate microfacies. These microfacies are: mud stone/wackestone, wackestone, wackestone/packstone and packstone. Dissolution of skeletal grains and infilling with iron oxides and neomorphic calcite are the dominant diagenetic features. Key-words: Dakhla Shale, Esna Shale, Tarawan Chalk, Nile Valley Facies, clay minerals. gross lithologic similarities and they belong to the same Introduction facies type (Nile Valley Facies) (Issawi, 1972). The lower Paleogene succession of the Nile Valley Lower Paleogene sediments were deposited over facies has attracted the attention of several geologists. large areas of Egypt as far south as the border of Sudan. Most of the previous studies were interested in the litho They are widely distributed in the Eastern and Western and biostratigraphic aspects (Said, 1962; 1990; Strougo, Deserts, Nile Valley, Gulf of Suez area and Sinai. 1986; Speijer and Schmitz, 1998; Faris et aI., 1999 and Among these areas, the lower Paleogene sediments at others) and the mineralogical characteristics of the suc the Central Nile Valley and the Kharga Oasis display cession (Marzouk, 1985; Soliman et aI., 1989; Tantawy 40 Texture, mineralogy and microfacies of the Lower Paleogene succession, central Egypt et a!., 2001). However, no previous studies have been uously except for short intervals (Said, 1990). done on the textural characteristics and the carbonate The lower Paleogene succession of the Nile Valley contents of the lower Paleogene sediments in this area. facies forms a tripartite subdivision arranged in ascend The present study aims at discriminating the lower ing order as the upper Dakhla Shale (Said, 1962), the Paleogene shales on the basis of their textural and min Tarawan Chalk (A wad and Ghobrial, 1965) and the Esna eralogical attributes, identifying the different microfa Shale (Zittel, 1883) (Fig. 1). These sediments were cies of the carbonate rocks, shedding light on the pale deposited in inner to middle shelf settings. oproductivity and paleoclimatic conditions that prevailed The Dakhla Shale was first described by Said during Early Paleogene time and their influence on the (1962) to describe the 130 m of shale succession over sediment composition. lying the upper Cretaceous phosphate bearing the Duwi Formation and underlying the Tarawan Chalk at the Mut Background and Lithologic Subdivisions location, Dakhla Oasis. Generally, the age assigned to the Dakhla Shale is Late Cretaceous - Paleocene (Said, Said (1962) subdivided Egypt into two major tec 1962; Faris et a!., 1999). The basal part of the Dakhla tonic provinces, the tectonically deformed Unstable Shale (Upper Cretaceous-lowermost Paleocene) is not Shelf to the north (approximately to the North of lati represented herein. The studied upper part of the tude 28°) and the nearly horizontal and less deformed Dakhla Shale consists of 32-46 m of offshore fossilif Stable Shelf to the south. Intracratonic sedimentary erous greenish grey, calcareous shale, marl and argilla basins were developed on the southern Stable Shelf, ceous limestones. The studied interval of the Dakhla whereas pericratonic or rift basins were formed on the Shale ranges from Early Paleocene (NP2) to Late northern Unstable Shelf (E1- Hawat, 1997) Paleocene (NP7/8) (Faris et a!., 1999). In the Stable Shelf of Egypt, thick Phanerozoic suc The Tarawan Chalk (Awad and Ghobrial, 1965) cession was deposited in two broad intracratonic consists of fossiliferous, partly marly or chalky, yel depocenteres, the Upper Nile and the Dakhla basins. lowish white limestone of an outer shelf facies. The They are separated from each other by the Kharga and boundary between the Tarawan Chalk and the underly Dakhla basements, which are aligned in N -S and ing Dakhla Shale forms a significant regional discon WNW - ESE directions, respectively. These basins have formity recognized throughout southern and central evolved as a result of structural differentiation and sub Egypt. It corresponds to the boundary between the sidence of the rigid cratonic plate (El-Hawat, 1997). planktonic foraminiferal zones P3 and P5, and is Precambrian crystaJline basement highs surround these referred to as the .. Velascoensis Event" (Strougo, 1986; basins from the southern and eastern sides and they were Hermina, 1990). The Tarawan Chalk varies in thick the main source for terrigenous sediments during ness from 13 to 36 m. It is of Late Paleocene age (NP5 Phanerozoic time. During Late Cretaceous, the Dakhla NP7/8 Zones) (Faris et a!., 1999). and Upper Nile basins were joined as a result of relief The Esna Shale (Zittel, 1883) is a thick succession inversion and subsidence of the Kharga high (El - Hawat, of green shale, enclosing marl and limestone intercala 1997). tions and lies between the two carbonate units, the The lower Paleogene sediments in the Upper Nile Tarawan Chalk at the base and the Thebes Formation at basin disconformably overlie the Cretaceous sediments the top. Like the Dakhla Shale, the Esna Shale is and are separated from it by thin intraformational con inferred to be accumulated mostly in inner to middle glomerate carrying reworked Cretaceous fossils (Said, shelf environments (Said, 1990). The age of the Esna 1990). Strougo (1986) related the stratigraphic rela Shale is Late Paleocene-Earl Eocene (NP7/8-NP11) tionships in the south and central Western Desert and (Faris et a!., 1999). in other areas to a large scale syndepositional tectonic disturbance, including faulting, which had led to marked Materials and Analytical Techniques changes of depositional pattern and he named this as Velascoensis Event. The materials of the present study were collected The Paleogene was ushered in by a transgression, from two different areas in Central Egypt; the Qena which pushed its way across the southern borders of Esna stretch on the eastern border of the Nile Valley Egypt into north Sudan. The maximum transgression and the Kharga Oasis. Four lithostratigraphic sections occurred during the Late Paleocene. After the Paleocene, (Fig. 1) were measured and sampled at G. El-Sheikh the sea kept retreating towards the north almost contin- Eisa (Qena region), G; El-Shaghab (Esna region), G. I. M. GHANDOUR et al. 41 32° ~N @ East Sahara Craton ,J.... ~~.~~ @ Nubian Shield \ ~~ ro,.... ~ Highs 30° I o?' ~~ \~ ~~ I~ ~ ...K"Troughs 1;;..- '0 Bahariya :~ Oasis LITHOLOGY 28° I Western I I Desert Shale 1 I Farafra Marl I Oasis 26° Argillaceous o..........80 [JlZJKharga limestone km ·!~asis 24 @ II @ ·~---_~ ~YYE!~_---- SUDAN 26° 28° 34 30 m 20 10 40 39 o 34 - 35 35 23 30 30 30 Esna Shale 25 25 25 20 20 20 - 16 16 ~ H;:£i5:!r:1:;5-------.J ~ Chalk 12 IS 10 10 Dakhla Shale 9 8 10 'rJ··~··~- •••••• 1 3 2 I Fig. 1 Location map and the lithostratigraphic sections at the studied locations. 1) G. Teir/Tarawan, 2) G. Um El-Ghanayum, 3) G. El-Shaghab and 4) G. EI-Sheikh Eisa. The relative age and calcareous nannofossil zones are modified after Faris et al. (1999). The numbers at the right of the stratigraphic sections indicate sample numbers. 42 Texture, mineralogy and microfacies of the Lower Paleogene succession, central Egypt Urn EI-Ghanayum and G. Teir/Tarawan (Kharga Oasis). sand fraction is determined by 63.um wet sieving, dried A Total of 165 samples were collected for the present and weighed. The mud fraction (silt and clay) was then work. The lithology, the number of samples and their placed in 1000 ml cylinder and the distilled water was locations in each section are shown in Fig. 1. The stud added until the volume was exactly 1000 m!. After ied sediments were analyzed for their textural (42 sam checking the temperature, several withdrawals at spe ples), mineralogical (18 samples) and carbonate micro cific times were taken (Folk, 1968). facies characteristics (13 samples). In addition, the The XRD analyses were carried out using a carbonate contents of the studied sediments were also RIGAKU RAD-IX -ray powdered Diffractometer with determined.